Novel alibi could be immune to challenge (10)
I believe the answer is:
inviolable
'immune to challenge' is the definition.
(unable to be challenged or violated)
'novel alibi could be' is the wordplay.
'could be' indicates anagramming the letters.
'novel'+'alibi'='novelalibi'
'novelalibi' with letters rearranged gives 'INVIOLABLE'.
(Other definitions for inviolable that I've seen before include "Sacred" , "not to be discredited" .)